The present invention relates to backpacks in general and specifically to a method for centering the load of a backpack. Specifically, when a backpack wearer twists and/or tilts his torso, he changes the distances between his shoulders and hips. This tilting and/or twisting movement causes a backpack's center-of-load to move with the wearer for two primary reasons: (1) an increase in shoulder-to-hip distance exerts a pull on a fixed length shoulder strap and hence the backpack, and/or (2) a decrease in shoulder-to-hip distance creates slack in a fixed length shoulder strap and hence the load of the backpack on the shoulders shifts to a single shoulder. This movement of the center-of-load can cause the wearer to become unbalanced. Consequently, this movement of the center-of-load can be uncomfortable because strain is put on the back and shoulder muscles to maintain balance. More importantly, this movement of the center-of-load can be dangerous because it can cause the wearer to fall. In fact, situations which can cause the wearer to significantly twist and/or tilt his torso (e.g. traversing or climbing steep slopes, crossing talos slopes, crossing boulder fields, stepping over logs, crossing streams, crossing uneven or slippery terrain, cross country and alpine skiing, or even walking through dense vegetation) are precisely the kinds of situations where wearer balance is crucial and falls can be the most serious.
The present invention provides a backpack with left and right shoulder straps that change length in response to wearer torso twist and/or tilt to compensate for the tendency of these motions to shift the backpack's center-of-load. The principle behind the present invention is that a shoulder strap which adjusts its length in response to a change in shoulder-to-hip distance prevents the pull that would result for a fixed length shoulder strap when the shoulder-to-hip distance increases and removes the slack that would result for a fixed length shoulder strap when the shoulder-to-hip distance decreases. Thus, the present invention compensates for motion that tends to shift a backpack's center-of-load while maintaining a roughly even distribution between the two shoulders of the backpack's load on the wearer's shoulders. The present invention further provides a means to adjust the overall length of each shoulder strap so as to compensate for differences in left side and right side shoulder-to-hip distances that exist in the absence of wearer motion.
In a preferred embodiment, the lower ends of the left and right shoulder straps are standard backpack webbing straps which are joined to form a continuous strap, and pass through a channel on the bottom panel of a backpack. The lower end of the shoulder strap passes between the channel base, which is comprised of a low friction material, and the channel cover which is cupped with the concave side facing the joined webbing straps. In such an embodiment, the joined webbing straps slide within the channel such that one shoulder strap lengthens under the pull of wearer torso twist and/or tilt, while the other shoulder strap correspondingly shortens to take up slack.
In another preferred embodiment, the lower and/or upper ends of the left and right shoulder straps are made of an elastic material. In such an embodiment, the ends of a shoulder strap stretch when the shoulder-to-hip distance increases to prevent a pull on a backpack or contract when the shoulder-to-hip distance decreases to remove slack and prevent an unbalancing distribution of a backpack's load.
In another preferred embodiment, the lower ends of the left and right shoulder straps are wound around cylinders that are under radial tension such that a cylinder takes up slack in a strap. Such a device may comprise a single cylinder or a multiplicity of cylinders and is referred to as an uptake cylinder herein. In such an embodiment, an increase in shoulder-to-hip distance causes the uptake cylinder to reel out more strap and lengthen the strap, while a decrease in shoulder-to-hip distance allows the radial tension of the uptake cylinder to reel in slack and shorten the strap. Further, in such an embodiment, the left and right shoulder straps may be wound around separate and independent uptake cylinders or the same uptake cylinder.
Other embodiments of the above further comprise a means to adjust the overall length of each shoulder strap so as to compensate for differences in left side and right side shoulder-to-hip distances that exist in the absence of wearer motion. This overall shoulder strap length adjustment means further enables the above embodiments to distribute the load of the backpack on the wearer's shoulders in a roughly even manner and to adjust the length of the shoulder straps in response to wearer torso tilt and/or twist.